1. Field of the Invention
This invention relates to molding of plastic, and more particularly relates to syndiotactic polypropylene and a process for its molding.
2. Background of the Invention
Molding is a process in which melted or plasticized polymeric material is forced into a mold where it is held until removed in a solid state having a shape which duplicates the cavity of the mold. The process consists of three basic operations: (1) raising the temperature of the plastic to a point where it will flow under pressure into the mold cavity, (2) allowing the plastic to solidify in the mold (conventionally referred to as hold time) while maintaining pressure during removal of heat, and (3) opening the mold to eject the plastic permanently frozen in the shape of the mold. The productivity of the molding operation depends on the speed of melting the plastic, the speed of injection and ejection, and the hold time required to cool and solidify the product in the mold. Thus, in the molding art, cycle time is defined as the time required for one complete operation of a molding press from closing time to closing time. In most molding operations, the largest part of the cycle time is the hold time, which generally accounts for up to 80% of the cycle time, and may be as high as 95%.
Polypropylene (PP) has long been used in molding and extruding operations for articles such as containers and films for the food packaging industry, and is known to exist in three forms. In isotactic polypropylene (IPP), the methyl groups are attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the polymer chain. Syndiotactic polypropylene (SPP) has the methyl groups attached on lo alternating sides of the polymer chain. Atactic polypropylene (APP) has the methyl groups attached randomly with respect to the polymer chain. APP is essentially a waxy product of low melting point whereas SPP and IPP are crystalline and of higher melting point.
All conventional molding and extruding grades of PP are substantially isotactic and are synthesized using Ziegler-Natta polymerization catalysts. Depending on the particular polymerization catalyst and conditions, these products may contain limited amounts of monomeric units having other configurations.
SPP, although known for many years in various degrees of syndiotactic purity, has been only a laboratory curiosity until recently. A new crystalline SPP of exceptionally high syndiotactic purity prepared by a process using syndiospecific metallocene catalysts is disclosed by Ewen et al. in U.S. Pat. No. 4,892,851, incorporated herein by reference. The product is stated to have a higher melting point and lower heat of crystallization that IPP. European Patent Application 428,972 discloses a method to improve the transparency of a sheet prepared with the Ewen et al. SPP.
In copending application Ser. No. 08/054,476 filed on Apr. 30, 1993, a syndiotactic polypropylene composition of greatly reduced cycle time is disclosed.